What three-phase power is
Most industrial sites receive alternating current in three phases. Think of three separate alternating currents of the same frequency, each offset from the others by one third of a cycle. Because the offsets are evenly spaced, the combined supply delivers power smoothly rather than in pulses.
For motors this has a useful consequence. The three windings in a motor stator, fed by the three phases, create a rotating magnetic field without any extra starting parts. A single-phase motor needs a capacitor or other starting method to get the same effect. Three-phase motors therefore tend to be simpler, more compact for a given power, and more efficient than comparable single-phase ones, which is why they dominate in industry.
The speed of the rotating field
The rotating field turns at a speed called the synchronous speed. It depends on supply frequency and the number of magnetic poles, using the formula 120 times frequency divided by poles. For a 60 Hz supply with four poles, that is 120 x 60 / 4 = 1,800 revolutions per minute. On a 50 Hz supply, a four-pole motor gives 120 x 50 / 4 = 1,500. An induction motor turns slightly slower than this under load; the difference is called slip.
Main types
- Squirrel-cage induction motors. The workhorse of industry. The rotor is a cage of bars with no electrical connection, so it has no brushes or slip rings and needs little attention. A broader view of different industrial motor families is covered in another article on this site.
- Wound-rotor induction motors. The rotor has windings brought out through slip rings, allowing resistance to be added for high starting torque or some speed control. They need more maintenance because of the rings and brushes, and are less common today.
- Synchronous motors. The rotor turns at exactly the synchronous speed. They are used for steady, large loads, and can sometimes help with power factor.
- Permanent-magnet and servo-type three-phase motors. These are used with drive electronics for precise control, as in servo motors for machine tools and robots.
Typical applications
Three-phase motors drive pumps, fans, compressors, conveyors, mixers, crushers and machine tools. Matching the motor to the load includes checking the required torque, how often it starts, the ambient temperature, the enclosure type, and whether the motor will run on a variable speed drive. The advantages of using electric motors in a facility are discussed elsewhere; the point here is that sizing and enclosure choices have to suit the actual duty.
Starting methods vary. Small motors are often started directly across the line, while larger ones use reduced-voltage starters or drives to limit inrush current and mechanical shock. Starters and protection are usually grouped in motor control centers.
Reading the nameplate
The motor nameplate lists voltage, current, frequency, speed, power rating, insulation class and enclosure type, among other items. It is the starting point for checking that supply, protection and the connected load are compatible. The ideas are explained under equipment data plates.
Routine care
A sensible program is mostly observation and cleanliness, scheduled in line with the manufacturer’s manual:
- Keep vents and cooling fins clear. Heat is a common enemy of insulation.
- Listen and feel for changes in noise, heat and vibration.
- Lubricate bearings as specified, avoiding over-greasing, which can overheat them.
- Check mounting bolts, couplings and shaft alignment.
- Watch for signs of moisture, dust or oil getting into the motor.
- Compare running current with nameplate values and with earlier readings.
- Where equipment and expertise allow, include insulation resistance testing and thermal imaging in a preventive maintenance routine.
Bearing wear patterns and vibration trends often give the earliest warning of trouble.
Supply problems that hurt motors
Unbalanced voltages between phases, a lost phase, low or high voltage, and distorted waveforms can cause overheating and early failure. These issues relate to power quality and are typically diagnosed with proper instruments.
Safety and who does the work
Motors are connected to hazardous voltages and drive moving machinery that can start unexpectedly. Electrical work, including terminal box work, testing and reconnection, should be carried out by trained and qualified personnel, with lockout and tagout style isolation, the manufacturer’s instructions and local regulations in force. This article is general information only.